I assume you have checked out the packages kableExtra and flextable?
If they can't do the job, is it possible in your application to edit the raw HTML code?
library(tidyverse)
tb <- tibble(text=c("this image is awesome", "this image is awful"), likes=c(34,8))
tb
library(knitr)
t1 <- kable(tb, format="html")
search_word <- "awesome"
replace_string <- paste0("<b>", search_word, "</b>")
t2 <- gsub(search_word, replace_string, t1)
writeLines(t1, con="test_t1.html")
writeLines(t2, con="test_t2.html")
HTH